Add PAUSE instruction to RISC-V core::arch

Author: luojia65

crates/core_arch/src/riscv/mod.rs

@@ -1 +1,10 @@
 //! RISC-V intrinsics
+
+/// Generates the `PAUSE` instruction
+///
+/// The PAUSE instruction is a HINT that indicates the current hart's rate of instruction retirement
+/// should be temporarily reduced or paused. The duration of its effect must be bounded and may be zero.
+#[inline(always)]
+pub fn pause() {
+    unsafe { asm!(".word 0x0100000F", options(nomem, nostack)) }
+}